The present invention relates to a method for hydrating a material based on hemihydrate gypsum to obtain a mold mainly composed of gypsum dihydrate, suitable for castmolding pottery More specifically, the present invention pertains to a method for preparing a mold of gypsum, utilizing a technique for hydrating hemihydrate gypsum, which comprises stirring a mixture of hemihydrate gypsum cooled to -40.degree. to 5.degree. C. and water to form a slurry of gypsum in water having a temperature of -5.degree. to 10.degree. C., pouring it into a water-impermeable mold, maintaining it therein until the setting reaction thereof is completed and then releasing the product from the mold.
As a mold material for casting pottery, it is common to use gypsum because of its good water absorption properties. A method for manufacturing a mold of gypsum (hereunder referred to as "gypsum mold") in general comprises the steps of mixing powdery hemihydrate gypsum with water to form a slurry of gypsum in water, pouring the slurry into a waterimpermeable mold, maintaining it therein until the setting reaction thereof is finished and then releasing the set product from the mold. In such methods, hemihydrate gypsum is converted to gypsum dihydrate in accordance with the hydration reaction represented by the following reaction formula: EQU CaSO.sub.4.1/2H.sub.2 O+3/2H.sub.2 O.fwdarw.CaSO.sub.4.2H.sub.2 O
In this reaction, hemihydrate gypsum is first dissolved in water and is thereafter recrystallized as gypsum dihydrate. The crystal of gypsum dihydrate grows in the form of needles and, therefore, capillaries are formed, in the resultant gypsum mold, from voids formed between crystals of gypsum dihydrate. Such high water absorption properties of the gypsum mold are attributable to its high capacity of capillary action.
Making use of these water absorption properties of the gypsum, pottery can effectively be cast-molded from a mold made of gypsum. Water in slurry for casting pottery is taken up by such a gypsum mold when the slurry is poured into the gypsum mold and then is held therein for a prescribed period of time. The water component of the slurry is first reduced due to absorption by the gypsum mold and the slurry is then solidified to form cake and to thereby form a product having the desired shape. In this regard, the time required for the gypsum mold to absorb water from the slurry, in other words the time required for casting a cake, accounts for a major part of the overall time required to form pottery. Thus, to improve the productivity of pottery, it is of great importance to reduce this time.
The casting rate of the cake can be expressed as the casting rate constant K defined as follows: EQU K=L.sup.2 /t
In this formula, K represents the casting rate constant (cm.sup.2 /sec), t is the casting time (second) and L represents the thickness of the cake (cm).
K is a constant which is determined from the water absorbing capacity of the gypsum mold and the properties of the slurry for casting pottery and is always a constant irrespective of the casting time t. Therefore, for comparing the water absorbing capacity between different gypsum molds, it is necessary to determine the values K thereof obtained by pouring, into the gypsum molds, slurry for casting pottery having the same properties and to then compare them with each other. As the value K of the mold increases, the casting time of the gypsum mold becomes shorter and thus a gypsum mold with a high value K makes it possible to manufacture pottery with high efficiency.
The casting rate constant K of the gypsum mold is determined by the strength of the capillary action thereof. Thus, the higher the capillary action, in other words, the finer the crystals of the gypsum dihydrate constituting the gypsum mold and the smaller the diameter of the capillaries formed from the voids between the crystals of gypsum dihydrate, the larger the value K. In this connection, this is true only on the premise that the resistance to water penetration of the cake is greater than that of the gypsum mold. Therefore, if a special slurry which forms a cake having a very low resistance to water penetration is used, the diameter of the capillaries cannot be reduced so much. Otherwise, it is sometimes observed that the resistance to water penetration of the cake becomes so large that it can no more be disregarded as compared to that of the gypsum mold and this leads to reduction of the value K.
It is considered that the size of the gypsum dihydrate crystals varies dependent upon the water content of the gypsum slurry from which the gypsum dihydrate is formed, the speed at which the gypsum slurry is stirred and stirring time as well as on the impurity level or another factors. Besides, the temperature of the gypsum slurry is also an important factor which cannot be disregarded. The lower the temperature of the gypsum slurry, the finer the gypsum dihydrate crystals formed and hence the greater the value K. On the contrary, as the temperature of the gypsum slurry becomes lower, the setting requires a longer time and hence the workability is likely to be impaired. For this reason, the temperature of the gypsum slurry (i.e., the temperature thereof after the completion of stirring of the mixture of hemihydrate gypsum and water) generally adopted ranges from 10.degree. to 20.degree. C. and in particular is preferably around 15.degree. C. Taking into consideration, for instance, the increase in temperature of the slurry due to heat of hydration during stirring, it is not possible to obtain a gypsum slurry of around 15.degree. C. by simply mixing water of ordinary temperature and gypsum. Thus, the water is cooled prior to mixing it with hemihydrate gypsum to form a gypsum slurry of a desired temperature. This is because cooling of the water is more effective than cooling of the gypsum since the latter is a powder with a small specific heat that is only on the order of 1/5 that of water.